This invention relates to optical fiber Bragg gratings and, in particular, to a fiber Bragg grating provided with an index-matched polymer coating to reduce short wavelength cladding mode loss.
Optical fiber Bragg gratings are critical wavelength filtering components in WDM communication systems. In these applications, the grating is typically used as a reflective filter. Incident light within the stopband of the grating is strongly reflected whereas light outside the stopband is transmitted. An ideal Bragg grating would possess a rectangular amplitude filter function; the reflection would be unity within the stopband and reflection and loss would be negligible outside the stopband.
In practice, an important limitation on a realistic optical fiber Bragg grating is cladding mode loss on the short wavelength side of the main reflection band. This short wavelength cladding mode loss is caused by coherent coupling from the grating into backward propagating cladding modes. The cladding mode loss is seen in the transmission spectrum as sharp resonances, approximately periodically spaced, on the short wavelength side of the Bragg resonance. The magnitude of the loss scales approximately with the square of the strength of the grating, and the loss is dramatically exacerbated when many gratings are cascaded. It thus imposes strict limitations on the design of optical networks that use gratings.
Several approaches have been proposed for reducing Bragg grating coupling into claddings. A first approach uses a depressed cladding design. See L. Dong et al. xe2x80x9cOptical Fibers with Depressed Claddings for Suppression of Coupling into Cladding Modes in Fiber Bragg Gratings,xe2x80x9d IEEE Photonic Technology Letters, Vol. 9, page 64-66 (1997). A conventional fiber core is surrounded by a down-doped region, typically using boron to achieve the down doping. The depressed cladding region suppresses the overlap of lower order cladding modes with the core. The transverse oscillations are stretched in the depressed cladding region, since the traverse resonance condition is associated with the optical path length (distance times refractive index). This approach has been demonstrated with moderate success. But it is limited by the amount that the index can be reduced in the depressed cladding region.
A second approach is to increase the offset of the cladding mode loss from the Bragg resonance. This is achieved by increasing the core refractive index such that the effective core mode index is substantially above that of the lowest order cladding mode. In practice, this means that the core mode has an effective index substantially above the refractive index of silica, since the lowest order cladding mode has an effective index very close to the refractive index of silica. As a result the cladding mode resonances are offset from the Bragg resonance. Various groups have demonstrated this effect, where typically a fiber with xcex94xcx9c2%, and a core diameter of dxcx9c2 xcexcm, is used, resulting in an offset of xcx9c8 nm. Although the principle has been demonstrated, the usable bandwidth is still limited by the onset of cladding mode loss. In addition there is a significant splice loss penalty incurred due to mode mismatch between the grating fiber and the transmission fiber.
The cladding mode loss can also be reduced by incorporating photosensitive material into the cladding of the fiber. (See, e.g., E. Delevaque et al. xe2x80x9cOptical Fiber Design for Strong Gratings Photoimprinting with Radiation Mode Suppression,xe2x80x9d OFC ""95, PD5, USA, 1995 and K. Oh et al., xe2x80x9cSuppression of Cladding Mode Coupling in Bragg Grating Using GeO2xe2x80x94B2O3 doped Photosensitive Cladding Optical Fiberxe2x80x9d, Electronic Letters, Vol. 35, page 423-424 (1999)). In this case, after UV exposure the grating region extends into the cladding region. The reduction in the cladding mode loss follows from the orthogonality condition. Hence if the core and the cladding have the same UV sensitivity, there is no blaze and the exposure through the fiber is uniform. Thus the grating will give negligible coupling to the cladding modes. A disadvantage of this scheme is a net reduction in the grating strength due to absorption in the photosensitive cladding region. There is also an increased coupling to asymmetric modes because of the increased asymmetry in the region where these modes have a large mode field strength.
Cladding mode loss is also reduced by the polymer coating conventionally applied around the cladding for environmental protection. The standard polymer coatings are lossy and have a refractive index greater than that of silica. (Typically npolymer=1.51; nsilica=1.45). In this case, the cladding modes extend into the polymer where they are absorbed, and thus coherent feedback into the fiber is reduced. The reduced cladding mode loss is acceptable for many applications but can still limit the number of devices that can be cascaded.
Accordingly, there is a need for an improved fiber design which can effectively eliminate cladding mode loss in fiber Bragg gratings.
In accordance with the invention, an optical fiber Bragg grating comprises a length of glass optical fiber having a core, a Bragg grating formed along the core, a glass cladding and a polymer coating on the cladding having an index of refraction matched to that of the cladding. Such index matching can reduce the cladding mode loss by a factor of four over current levels. A preferred polymer coating material comprises fluorinated urethane acrylate.